1. Field of the Invention
The present invention relates to a speaker and more particularly to a speaker which is adapted for use as a center woofer in a 3-Dimensional ("3D") sound system such as a stereo.
2. Description of the Related Art
A 3D sound system such as a stereo produces a low frequency sound range of about 100 to 150 Hz or less to provide a 3D sound effect. Such low frequency sound is usually produced by one speaker functioning as a woofer. This one speaker for reproducing the low frequency sound usually includes a first sounding body for generating sound in accordance with a left channel signal and a second sounding body for generating sound in accordance with a right channel signal.
FIG. 3 shows a conventional speaker which functions as a woofer for generating a signal in which both right and left channels of the low frequency sound range of about 100 to 150 Hz or less are combined.
The speaker 6 includes a main body 1, a partition 2 provided within the main body 1, two sounding bodies 3 and 4 attached to the partition 2 and a duct 5 provided at the lower part of the front surface of the main body 1. The sounding bodies 3 and 4 are attached to the main body 1 such that the sounding bodies are arranged to transmit sound toward and through the duct 5. Although not shown specifically in FIG. 3, a sound in the low sound range of the right and left channels of less than 100 to 150 Hz is output when a signal of the left channel is input to the sounding body 3 and a signal of the right channel is input to the sounding body 4.
At this time, the sounds emitted directly from the sounding bodies 3 and 4 are not emitted to the outside of the main body 1. Only sounds having a frequency which is equal to a resonant frequency of the duct 5 and the main body 1 are emitted to the outside of the main body 1.
However, because a sounding body, e.g. a cone plate, which allows a large amplitude sound to be obtained is used in the prior art speaker described above in order to obtain a large output in the low sound range, the overall size of the speaker must be very large.
It is possible to form the sounding body to comprise a plate-like piezoelectric element to reduce the size of the speaker and eliminate the size problem. However, when such a piezoelectric element is used as the sounding body in the prior art speaker described above, the output in the low sound range is insufficient because the amplitude of sound generated by the piezoelectric element is small as compared to the amplitude of the sounding body comprising the cone plate.
An overall area or size of the piezoelectric element may be increased to increase the amplitude of the piezoelectric element. However, such a solution inevitably and significantly increases the necessary overall size of the speaker.
Further, because the low frequency sound output by the conventional speaker in FIG. 3 depends on a resonance frequency existing between the duct and the main body and there is only one resonance frequency in the prior art speaker, a sufficiently low sound output level can not be obtained for a low sound range which has a frequency that is different from the resonance frequency of the conventional speaker. Therefore the sound output by the conventional speaker is insufficient in the entire lower frequency range.